Electrowetting has become an attractive modulation scheme for a variety of optical applications due in part to a desirable combination of high brightness and contrast ratio, a large viewing angle, and a fast switching speed. In addition, the power consumption of electrowetting displays is relatively low because they do not require front or backlighting. For example, electrowetting has been used to provide optical switches for fiber optics, optical shutters or filters for cameras and guidance systems, optical pickup devices, optical waveguide materials, and video display pixels. The term “electrowetting” describes the effects of an electric field on the contact angle of a liquid with a hydrophobic surface. With an electric field, the liquid distributes over, or wets, a surface that initially repels the liquid resulting in a change in the spectral properties of a device. When the electric field is removed, the contact angle increases and the liquid contracts into an area whereby the spectral properties are returned to the initial state.
Colored immiscible fluids are an indispensible part of electrofluidic and electrowetting devices, where reproduction of visual information and effects are required for the application. Conventional electrowetting devices typically have a colored oil that forms a film over an insulating fluoropolymer. This colored oil film imparts a visible color to the device. When a voltage is applied between a water layer situated above the oil film and an electrode beneath the insulating fluoropolymer, the oil film is disrupted as water electrowets the surface. The disrupted oil film no longer provides color to the device. Once the voltage is removed, the oil preferentially wets the insulating fluoropolymer, the oil film is reformed, and the color is again evident.
In general, the colorant can be a dye or a pigment. Historically, dyes have been the colorant of choice for various digital applications such as inkjet inks, color filters, and electrowetting devices. Dyes however have certain disadvantages including poor light and weather fastness. Other disadvantages include high cost, especially for purified forms, inadequate solubility in non-polar solvents, low resistance to bleed, and/or a lack of opacity. In applications where dyes have been employed as coloring agents, organic pigments have been finding increased utility in recent years for due to desirable light fastness and resistance to solvents and bleed. Such applications include, for example, inks for writing instruments, in which water or oil-soluble dyes have been used as coloring agents; and colorants for plastics, in which oil-soluble dyes have been used as highly transparent colorants. There is also increasing demand for pigments as coloring agents for LCD color filters, toners, and ink jet inks.
Many devices that work with electrowetting use a combination of water and oil. However, the physical properties of water, such as expansion at higher temperature and freezing point, limit the applications for such devices. While the problems associated with the use of water and conventional dyes are being addressed, there still remains a clear need for improved colored fluids for a variety of electrowetting and electrofluidic devices.
It would thus be beneficial to provide an improved colored fluid for electrowetting, electrofluidic, or electrophoretic devices that, for example, can enhance device performance and maintain a desired function over a preferred period of time.